This invention generally relates to projection apparatus and more particularly relates to an apparatus and method for forming a curved image from a substantially flat image source using a monocentric optical system.
In conventional projection apparatus, an image, provided from an essentially flat image-forming surface, is projected onto an essentially flat display surface. In film-based projection, for example, light is transmitted through a flat piece of film for projection onto a flat movie screen. Digital image-forming devices, such as transmissive and reflective Liquid Crystal Displays (LCDs) and Digital Micromirror Devices (DMDs) similarly produce an image on a flat surface. This allows projection apparatus employing these devices to use output optics that are similar to the projection optics employed in film-based projectors.
A number of optical systems, however, form images using curved surfaces, particularly optical systems of the immersive type that are designed to provide a wide field of view. One example system of this type is disclosed in commonly assigned U.S. Pat. No. 6,416,181 (Kessler et al.), incorporated herein by reference, herein referred to as the ""181 patent. In the ""181 disclosure, a curved mirror is employed, in combination with an imaging source, a diffusive surface, a ball lens, and beamsplitter, for providing a virtual image. The monocentric optical apparatus of the ""181 disclosure provides autostereoscopic imaging with large viewing pupils, a very wide field of view, and minimal aberration. In order to provide this type of imaging and take advantage of the inherent benefits of monocentric projection, the apparatus of the ""181 disclosure, given its source image formed on a flat surface must form an intermediate image having a suitable curvature.
Referring to FIG. 1, there is shown, extracted from the more detailed description of the ""181 disclosure, a portion of an image generation system 70 for providing an intermediate curved image 80 for projection in a stereoscopic projection system 82. Here, an image generator 74 provides a source image from a flat surface, such as from a reflective LCD. A relay lens 54 directs light from image generator 74 onto a diffusing element 32, so that a curved intermediate image 76 is formed on a diffusive surface 40. A ball lens assembly 30, cooperating with a beamsplitter 16, projects curved intermediate image 76 toward a front focal surface 22 of a curved mirror 24 to form intermediate curved image 80. Curved mirror 24 then provides a virtual image of intermediate curved image 80 for an observer 14.
As the ""181 disclosure points out, forming an intermediate image on a diffusive surface helps to overcome limitations imposed by the LaGrange invariant. A product of the size of the emissive device and the numerical aperture, the LaGrange invariant determines output brightness and is an important consideration for matching the output of one optical system with the input of another. Use of the diffuser with the ""181 apparatus is necessary because the image-forming device, typically a reflective LCD or other spatial light modulator, is a relatively small emissive device, measuring typically no more than about 1 inch square. Referring again to FIG. 1 and to the ""181 disclosure, in order to maximize the light output from image generator 74, it is necessary to provide a large angle of emitted light, using diffusing element 32, in order to adequately fill the pupil of the projection optical system (downstream from image generator 74 in the ""181 apparatus). Diffusive surface 40 is shaped to provide curved intermediate image 76 with the desired curvature for the projection optical system.
While use of a diffusive element provides a workable solution for forming a curved image, there are some drawbacks to using a diffusive component. In scattering light, diffusive optical elements necessarily cause some loss of brightness. Diffusive surfaces tend to increase graininess in an image and reduce contrast. Thus, while use of diffusive surfaces helps to surmount LaGrange invariant limitations, there is a need for a solution that provides an intermediate curved image and allows increased levels of brightness and high contrast. Brightness, expressed in units of nits or candelas/m2, is particularly important for providing displays that are readable under daylight conditions. As a rule of thumb, bright displays provide a luminance of about 250 nits or higher. Notebook LCDs, for example, typically provide luminance in the range of about 70 nits, unsuitable for most outdoor viewing. Automated Teller Machines (ATMs) typically provide luminance in the range of about 600 nits. Avionics displays provide from 900 to 1200 nits. For immersive imaging and simulation applications in particular, it is desirable to be able to provide a curved image having high brightness.
Thus, it can be seen that there is a need for a low-cost imaging subsystem that provides, from a substantially flat image source, a curved image having high brightness as an intermediate image for projection and display apparatus.
It is an object of the present invention to provide an apparatus and method for forming a curved image from a substantially flat image source. With this object in mind, the present invention provides an apparatus for forming a curved image comprising:
(a) a spherical mirror having a mirror center of curvature;
(b) a beamsplitter disposed between the vertex of the spherical mirror and the mirror center of curvature;
(c) an image source for providing image-bearing light to the spherical mirror;
the spherical mirror cooperating with the beamsplitter to form an intermediate image of the image source, the intermediate image having an image center of curvature; and
(d) a ball lens segment, centered at the image center of curvature, for forming the curved image from the intermediate image.
It is a feature of the present invention that it provides a curved image using a straightforward optical system, using relatively inexpensive reflective, beamsplitting, and projection optics.
It is an advantage of the present invention that it provides an optical system for forming a curved image that can be packaged in a compact fashion, allowing its use its use in desktop and other small-scale imaging applications.
It is a further advantage of the present invention that it allows the use of image source components having a large area, thereby allowing increased image brightness, even exceeding minimum brightness levels proposed for daylight displays.
It is a further advantage of the present invention that it forms a curved image without the need for a diffusive surface, thereby allowing the curved image to have higher contrast and minimizing graininess.